This invention relates to a process for the denitrification of water containing nitrogen in the form of nitrates and nitrites, which is at least substantially free of carbon, and wherein the water to be denitrified is passed through a reactor containing a carrier material for supporting denitrifying agents thereon. In another aspect, the invention relates to a method of preparing a carrier for use in the denitrification of water substantially free of carbon, and to the carrier itself.
In the prior art, it is known to eliminate nitrogen contained in sewage water in biological clarifier installations by nitrification and denitrification. In the process of nitrification, which primarily occurs only after extensive oxidation of carbon compounds present, and which requires low sludge loads, an adequate supply of oxygen and long sludge residence times, nitrogen in the form of ammonia is converted by autotrophic bacteria into nitrates and nitrites. In denitrification, nitrates and nitrites are reduced to elemental nitrogen under anoxic conditions. This reduction is effected by facultative anaerobic bacteria, e.g., facultative anaerobic heterotrophs with the principal genera being Pseudomonas, Micrococcus, Achromobacter and Bacillus, through an electron transport system whereby nitrates and nitrites are reduced by the use thereof as an electron acceptor. The denitrifying bacteria are thus capable of extensively oxidizing organic substrates without requiring molecular oxygen, and thereby acquire energy, and in turn, the denitrifying bacteria grow while utilizing organic matter for synthesis.
In the theoretical overall denitrification process, organic substances are oxidized by nitrites and nitrates into carbon dioxide and water. Consequently, in order to achieve extensive reduction of nitrates into molecular nitrogen, an adequate supply of organic carbon must be provided. In the prior art, one source of carbon has been raw sewage supplied to a clarifier installation. Another source of carbon has been a separately introduced carbon containing substrate, for example, methanol.
The denitrification is typically effected, in one case, in an anoxic zone preceding an activation zone and a nitrification zone, and which optionally contains a carrier material for supporting the denitrifying agents, i.e., microorganisms in this case, and is known as an attached growth process. An adequate supply of carbon is maintained, in the case wherein the anoxic zone precedes the activation and nitrification zone, by the influx of raw sewage, and the flow of sewage containing nitrates and nitrites and free of carbon, is recirculated from the nitrification zone into the anoxic zone. Alternatively, an anoxic zone is arranged following the nitrification zone, and also optionally containing a carrier material for the denitrifying microorganisms, and with a dosage of raw sewage containing BOD (biochemical oxygen demand) provided as the source of carbon.
However, the recirculating process requires large scale, energy intensive recycle conditions, and leads as the result of the placement in front of the anoxic zone, to a deterioration in the degree of nitrogen elimination in the sewage treatment plant taken as a whole. If the anoxic zone is inserted as a followup to the nitrification zone, the requirement of adding a carbon containing substrate is detrimental to the BOD.sub.5 (five-day biochemical oxygen demand) of the runoff, and furthermore, the precise metering required is difficulte to achieve because the effects of overfeeding are not seen until sometime after it has occurred. These two processes thus cannot be effectively and efficiently applied to the recovery of drinking water. For more details regarding the specific details of the denitrification process, see Wastewater Engineering, Treatment/Disposal/Reuse, 2d ed., Metcalf and Eddy Inc., McGraw Hill at page 727-734, whose disclosure is incorporated herein by reference.